The present invention relates to a sheet-like member having alignment marks and an alignment apparatus for the same, and more particularly to a reticle, mask and wafer and apparatus used therewith for manufacturing semiconductor circuits.
As a typical example of an aligner for automatically aligning plural objects, there is an aligner for manufacturing semiconductor devices, such as IC, (integrated circuits) LSI (large scale integrated circuits) and so on. Those IC and LSI are manufactured by superposing a number of complicated circuit patterns. The tendency is toward higher processing speed and higher density of patterns, the width of the circuit lines is continuously required to be smaller and smaller, and the accuracy of alignment is required to be higher and higher even to the extent of orders of sub-microns. To meet such requirements, there has been developed an aligner of step-and-repeat type, which is called a stepper. In a stepper, a pattern of a reticle is projected onto a wafer at a unit or reduced scale. Because of the limit of the design of the projection lens optical system, the projection area is necessarily limited or small so that the entire wafer surface cannot generally be exposed at one shot. Therefore, to cover the entire surface, the pattern is projected on a part of the wafer surface nd stepped to the next part and projected, and this is repeated throughout a wafer. With the increase of the size of the wafer, the number of steps required for one wafer increases so that the time required for processing one wafer increases. On the other hand, prior to each projection of a pattern, i.e., exposure of the wafer to the pattern, the reticle and the wafer must have been aligned. Therefore, how they are aligned is important from the standpoint of alignment accuracy and the period required for alignment. It is known, as OFF-AXIS alignment, to first correctly place one of wafer and mask at a predetermined position outside the exposure station and then move it toward the exposure position by a predetermined distance which is assured by a laser interferometer. This type of alignment process enables a high speed operation, but involves problems that the alignment cannot be directly confined at the exposure station; that it cannot meet a nonlinear local distortion which may be created in the wafer with experiences of wafer processing; and that the accuracy of the stage movement monitoring may affect the alignment.
There is a so-called TTL type apparatus wherein the wafer is observed through a projection lens adjacent the exposure position to align it with the reticle. This type of device can meet the local distortion of the wafer and can avoid the inaccuracy in the wafer stage movement so that a better alignment between the reticle and wafer can be expected.
For the TTL system, laser beam scanning is known for use in the alignment operation. An example thereof is described in a Japanese Laid-Open Patent Application No. 54-53562 which has been filed by the Assignee of the present application. FIG. 1 shows a schematic view of the device disclosed therein, for the sake of explanation. A single laser beam from a single laser beam source 1 is split or divided into two beams, which are then directed to lefthand and righthand objective optical systems 11, thus allowing detection of the displacement or degree of misalignment between the reticle 12 and wafer 13 at two positions. The two position detection allows two kinds of displacement, that is, X and Y direction (translational) displacement and .theta. (rotational) displacement to be corrected, by moving one of the reticle or the wafer relative to the other.
The optical system disclosed in FIG. 1 includes a condenser lens 2 for focusing the laser beam, a polygonal mirror 3, an f-.theta. lens 4 and a beam splitter 5. The laser beam emitted from the laser beam generator 1 is deflected for scanning by the polygonal mirror 3 and then is incident on the beam splitter 5 and other elements. The system further includes a field lens 6, a view field splitting prism 25 which is effective also to divide the scanning laser beam into two beams. Because of the dual functions, the prism 25 may be said to be a view field dividing and spatial dividing prism. The beam is passed through or reflected by a polarization beam splitter 7, a relay lens 8 and a beam splitter 9 and reaches the objective lens 11, by which it is imaged on the objects to scan the same. The system of optical elements extending from a pupil imaging lens 14 to a detector 18 constitutes a photoelectric detection system. The device further includes a chromatic filter 15; a spatial frequency filter 16 for blocking specularly reflected (by the reticle or wafer) beams but allowing scattered reflected beams to transmit; an illumination optical system having a condenser lens 17, a light source 19, a condenser lens 20 and a chromatic filter 21; and an observation optical system having an erector and an eye piece. The function and operation of those elements are explained in detail in the above-identified Patent Application, so that detailed explanations thereof are omitted for the sake of simplicity.
In this example, the deflected beam, deflected by the polygonal mirror 3, is divided in its deflection range, by the view field dividing prism 25 which is optically conjugate with the reticle 12 and wafer 13, thus using effectively the quantity of light of the laser beam. The deflection line is traverse to the edge of the prism 25. The respective beams divided out by the prism 25 are directed through the respective objective lens 11 to the alignment marks, and scan the same, respectively. The alignment scope having the microscopes has an additional important function, i.e., the observation of the alignment marks. The observation is one of the functions, particularly, in monitoring the state of alignment and initial setting of a reticle. For the observation optical system, it is desired that the images are observed in a natural and easy manner.
FIG. 2 shows the image view fields observed through the eye piece 23 in the arrangement of FIG. 1. In FIG. 2, reference numeral 31 depicts the view field dividing line provided by the edge of the view field splitting prism 25; 32, the scanning line of the laser beam; 33, the view field through the righthand side objective lens; and 34, the view field through the lefthand side objective. The laser beams scan the alignment mark areas in the direction connecting the righthand and lefthand alignment marks. The alignment marks play important roles in manufacturing semiconductor circuits, but do not provide any actual circuit patterns. So, after the wafer has been completely processed, the parts thereof having the alignment marks are the non-usable areas. For this reason, the area occupied by an alignment mark is desirably as small as possible, so as to provide a better yield.
FIG. 3 shows an example of a reticle or mask (hereinafter called simply a reticle). If the alignment marks are provided on the scribe lines between adjacent chips 101, they do not require any particular space, so that the above-described problem is solved. Since the scanning laser beam runs in the direction connecting the alignment marks, two alignment marks are arranged along this direction, that is, along and within a scribe line which is near the center of the reticle.
However, in the case of a so-called stepper type exposure and alignment device, inter alia, in the reduction stepper, it is possible that one reticle, in its entity, corresponds to one chip so that there are scribe lines only at the marginal area, that is, no scribe lines are near the center which would be better to accommodate the alignment marks as explained above.
FIG. 4 shows a reticle 12 having a pattern of only one chip 101, wherein the alignment marks are shown by reference numerals 102.
As will be understood from FIG. 4, the two alignment marks 102 are located on a line which is greatly spaced from the center of the reticle 12. Therefore, at the parts of the reticle 12 which are apart from the alignment marks 102, i.e., near the bottom in FIG. 4, the alignment is not very precise, as compared with the area near the alignment mark 102, i.e., the upper side.
The degrading of the alignment caused by the alignment marks 102 located along a line which is far from the center of the reticle 12 is significant when the reticle 12 has a pattern for one chip. This is also a problem when the number of chips whose pattern are formed on the reticle 12 is an odd number, as compared with the case an even number of chips being formed on the reticle 12, since there is no central scribe line in the former case.
In the case where the alignment marks are provided at the opposite peripherals of the reticle pattern area at the same latitude, and the alignment marks of the wafer are located correspondingly, one of several of the reticle alignment marks used with a certain exposure step, i.e., a certain shot, may be overlapped with a wrong one of the wafer alignment marks for the next shot, so that correct alignment for the next shot is not possible.
When a chip pattern on a reticle is projected to a wafer, the wafer is also exposed to the reticle alignment mark patterns. If such an exposed wafer is processed, by for example, development and/or diffusion, the alignment marks on the wafer may be more or less damaged (U.S. Pat. No. 3844655). The damage may be a serious problem, when the wafer thus processed is again subjected to an additional exposure to another pattern in alignment with the existing pattern, since the wafer has to be aligned again with a reticle.